1. Field of the Invention
The present invention relates to an image forming apparatus equipped with a fixing unit.
2. Description of Related Art
With respect to an input voltage value for a power supply device which is provided in an apparatus and supplies electric power supplied from a commercial power supply to each load in the apparatus, there is frequently caused a voltage drop, depending on a line impedance of commercial power supply, the state of installation of the apparatus and the state of power distribution. When the input voltage value for the power supply device goes down to be lower than the power supply operating limit voltage value, it sometimes happens that the power supply device not only fails to supply the prescribed power to the loads in the apparatus but also stops or goes wrong.
The image forming apparatus equipped with a fixing unit is provided with a heating power supply section for the purpose of supplying electric power to a heating means such as an induction coil or a halogen heater. The heating power supply section is required to provide higher output compared with other power supply devices, and is of the structure wherein the operating limit input voltage value is high, because a load for supplying electric power is a heat source.
Therefore, when an instantaneous stop or a voltage drop takes place for electric power supplied from a commercial power supply, the voltage value of electric power supplied from the commercial power supply becomes equal to or lower than the operating limit voltage value earlier than other power supply devices, and the heating power supply section turns out to be unable to supply electric power to the heating means. Accordingly, in the image forming apparatus, electric power is not supplied to the heating means from the heating power supply section, and thereby, a heat roller is not heated, the fixing temperature is lowered to cause fixing failures, which makes it impossible to obtain desired images.
Therefore, there have been an electrical apparatus to activate a protection circuit when input voltage coming from a commercial power supply is lowered, or when it is equal to or lower than the prescribed voltage value, and an image forming apparatus to activate a breaking means (relay) for electric power supply in abnormal temperatures of a fixing unit employing a halogen heater or a film heating system.
As a method to solve the problems stated above, there has been disclosed an electrical apparatus wherein a driving means is driven to activate a protection circuit when a power supply is not higher than the prescribed voltage value in the course of a standstill (standing by) of the driving means, and a printing operations is stopped when a power supply is lower than the prescribed voltage in the course of printing (for example, TOKKAI No. 2000-29579).
Further, there is disclosed an image forming apparatus (for example, TOKKAI No. 2003-57991) having therein an input breaking relay that breaks electric power supply to a heating means of a fixing unit having therein a heating means, a switching means that controls electric power supply to the heating means and a relay abnormality detecting means that detects abnormality of an input breaking relay based on signals coming from an input voltage means which detects voltage of the input breaking relay, wherein the switching means is driven based on output of detection by the relay abnormality detecting means.
However, TOKKAI No. 2000-29579 represents a method for stopping electric power supply for preventing troubles in the power supply device in the case of low voltage, and it is inconvenient in use, because printing operations are stopped immediately when voltage becomes lower than the prescribed voltage value in the course of printing. Further, TOKKAI No. 2003-57991 represents a method for preventing malfunctions of a relay circuit that breaks electric power supply and for preventing excessively high temperature of a heating roller by making the relay to be on and off properly when a fixing unit is in abnormality. In TOKKAI No. 2003-57991, therefore, the main body control section and a user are not conscious of a fear that the heating power supply section cannot supply sufficient electric power to the heating means because of low voltage abnormality to cause fixing failures.
In the fixing unit of an induction heating system, when electric power supply from the heating power supply section is stopped, electric power supplied to the induction coil is stopped, and therefore, an induction magnetic field for heating a heating roller cannot be generated and a temperature of the heating power supply section is lowered to cause fixing failures. Further, when input electric power of the heating power supply section is lowered, the electric power control to be supplied to the induction coil cannot supply high frequency electric current for generating the induction magnetic field for heating the heating roller, resulting in a fear that not only fixing failures but also malfunctions and a runaway of a circuit for driving the induction coil are caused.
When the aforesaid problems are caused, the main body control section of the image forming apparatus only judges that the heating power supply section is stopped and a temperature of the fixing unit is lowered, thus, there is a fear that electric power to the heating means is topped to cause temperature drop of the heating roller (fixing temperature drop) and to cause fixing failures.
The invention has been achieved, in view of the problems stated above. Namely, an object of the invention is to provide an image forming apparatus equipped with a fixing unit wherein an occurrence of fixing failures can be controlled in the case of input electric power abnormality of a heating power supply section. Further, another object is to provide an image forming apparatus wherein malfunctions of a fixing unit can be controlled when input electric power for a heating power supply section is abnormal, and it is convenient to use.